IF: BJT 2 Stage with AGC

A experimental exploring the traditional IF system @5/30  2012

Short story: After successfully built my first SuperHet receiver i decide to built an RF experimental system. this IF system is built for this. and I want to figure out why my IF stage seem un-stable,  and what's wrong with my transistor and my IF transformer.

[ please reading  the last paragraph to get the Best way for traditional IF system @6/25 2012 ]

AGC and bias for Q1 design process

1) choose Q1 quiescent current for Ie, use 1mA

2) choose bias current for Q1, for 1mA, assume β  at lease 200, suggest bias current through R1 for base of Q1 at least: (1mA/200)*10 = 0.05 mA

3) chose VOL resistor value: D3 will shunt some current assume we bias the D3 at 0.2V, to get 0.05mA current, VOL resistor should be (0.2V/0.05) K = 4k, use 3.3K for roughly  compensate D3's bias current. [detector load interference with Q1 bias design ]

4) Calc R11+R6: Q1 base voltage: 1mA*220R+0.6V = 0.88V, so R11+R6 should be: [0.88V-0.2V(voltage on VOL)]/0.05mA= 12K

5) chose R11, C7: make RC time about 100mS to 500mS, so get R11=3.3K, R6=10k ( a little bit big than R11+R6)

6) R5 calc: supply Asume 10V, Q1 base:0.88V, so R5=(10V-0.880V)/0.05mA ~= 180k, get 150K for more current.

This process rough match the actual test value, 10%~50% differentiation is acceptable.

First Try [ test method is wrong, keep this F.Y.I ]

Configuration: Antenna-C0, Mixer-J0, LO-H0

Test Instrument: sweep signal source

NOTE:

     All test by oscilloscope 10% toleration.

     IF stage 1's output contain strong LO signal, 455khz voltage read out by estimated.

My sweep signal source output minimal level is 45mVpp connect to Mixer-J0 which had a high input impedance. the result list here:

Condition& result:

LO: 2Vpp@1.8440Mhz

RF: 45mVpp@1.3692Mhz

Mixer output: 474.8khz 35mVpp [ @ Mixer-J0's secondary of IFT ]

LO leak without RF: 5mVpp [ @ Mixer-J0's secondary of IFT ]

IF n1/n2 ratio: 12:1

Stage 1: T1's secondary :

220R deg-ration resistor, disconnected R9

output: 85mVpp, gain: 20*log(85mVpp/35mVpp) = 6dB

R9 change to 47R boost:

140mVpp: gain 20*log(140mVpp/35mVpp)= 12dB

Stage 2:

stage 1 loaded output: 40mVpp to IF2 (impedance mismatch)

out 120mVpp, gain, 20*log(120/40) = 10dB

here is the problem for this test :

1) how come Stage1+ stage 2 less than stage 1 alone?

2) how come stage 1's Gain only 12dB? a typical Gain such circuit is about 30dB

some investigation for this show that:

1) If R9 is shorted to ground, Stage1 is easily oscillating bellowing  right image [ check if the IF stage oscillating:should pull input down by 10k resistor @6/25 2012  ]

2) band pass of the total IF stage is shown in left image ( my sweep works !)

3) RF signal seem too strong

@6/25 2012

R9/R4 severely degrade the gain, practically short R9 to ground the IF stage is keeping stable. I use wrong method to check if the stage is stable at when this article write( @5/30  2012). the right way is, pull the input to ground, then check if it's oscillating.   if you keep R9/R4 there, in door receiver is almost impossible [Configuration: Antenna-C0, Mixer-J0, LO-H0 ]

Eliminate the oscillation and reduce the RF power by 50dB attenuator

Configuration: Antenna-C0, Mixer-J0, LO-H0

Test Instrument: sweep signal source

NOTE:

     All test by oscilloscope 10% toleration.

     IF stage 1's output contain strong LO signal, 455khz Vpp is estimated.

 IF1 : IF amplifier Stage 1

IF2 : IF amplifier Stage 2

Input RF: 15mVpp

LO: 2Vpp

mixer output: non detectable: too small to the scope to estimate

disconnect R9:

IF1 output to IF2: 30mVpp

IF2 output: 250mVpp

IF2 gain: 18dB

Adjust R9 get Max Gain but keep IF2 not overloaded

IF1 output to IF2: 220mVpp

IF2 output: 1.1Vpp

IF2 gain: 13dB

Total Gain Changed[by change the R9]:

20*log(1.1Vpp/250mVpp)= 12dB

IF system gain guess:  50dB

@6/25 2012

  because I use wrong method to check if the stage is stable at when this article write( @5/30  2012). I found if short R9 to ground , that is, Q1 emitter AC short to Ground, the sound is good and keep sensitive, no unstable when it's on duty.  but don't  short  R4 to ground, Q2 will oscillate.

AGC

disconnect the AGC(R6 connected to GND),  indoor where full of EMI interference,  with a big single tune loop antenna[3meter x 3meter square], the receiver[ Antenna-C0, Mixer-J0, LO-H0 ] is overload everywhere in whole MW band.

this very simple AGC is worked,  even without the C7.  to hear it: (outdoor, 7 near stations)

AGC on both of IF stage

On a long antenna, signal might overload the IF stage, here is the enhancement AGC. AGC voltage on C7 from 0.4V~  -0.6V. @6/4/2012

Some Design consideration for AGC:

* Q1,Q2's bias current share with detector load resistor: the  VOL POT.  An solution is use Diode lower the VCC for Q1/Q2's base supply.

* the AGC voltage should get by testing,  this configuration we get  -0.6V,  for best AGC control range[assume R6 voltage drop always 0.6V],  Q1 base voltage vary from: [0.2V(D3 drop)+ 0.8V(R11+R6)] = 1V to  [-0.6V[C7]+0.6V[R6] ] = 0V, total AGC voltage change range around 1V

*On strong station, AGC make  bias of Q1 reached "OFF" state  so suggest that bias  Q1/Q2's base around  1.6V ~ 1.8V.

Following test not include the mixer, instead, 465khz signal feed directly to IF stage

465khz: input :           7.5mV      0.000000014w

               T2 output:    1900mV  0.001093939W

                total gain:    48dB

465khz input:               84mV

                T2 output:     2Vpp

                total gain:   27dB

AGC range:   48dB - 27dB = 20dB

Try Improve performance 

  [ indeed, all trying make almost no  progress!! @6/25 2012 ]

I tying get more gain by increase the transistor's quiescent current.

1. R9，R4 =>33R

2. R6, R7=> 12k

3. R8,R2=> 330R, 0.5mA, 2mA

but failed, it's became un-stable, the  symptom is like this:

           1. touch antenna  cause whistle

            2. touch any IF transformer cause whistle

            3. audio full of noise, whistle, hissing,  noise( tend to oscillation)

More trying............. list all of my hard work here:

@6/14 2012 get 63dB stable Gain

input: 1mVpp

output: (7.6*0.2)=1.52Vpp

total gain: 20*log(1520/1)= 63dB

AGC: voltage -0.2V, (without signal: .0.388V)

IF2 primary: 5.2*0.2*10=10.4Vpp, secodary:7.6*0.02*10=1.52Vpp,  

IF1 primay:  0.01*10*3.2=320mV,   secondary: 1.8*0.005*10=90mV

IF1:

     base 1.363  emitter: 0.628

     DC  degeneration: 300R

     AC  degeneration: 47R

IF2 :

    base 1.025

    emiiter: 0.3V

    DC degeneration:300R

    AC degeneration: 78R

Final comment for all trying above

  @6/25 2012

though we get more and more gain by increasing the current , but ,back to 5/20, I think I  can't remove the AC degeneration resistor R9/R4 for stable the IF stage.  actually the R9 is can be short to ground, because the receiver sound good, there is no symptom show it will oscillation. my mistake is that:

       I think it was UN-stable because it was oscillating if I keep input open. 

while IF system input is Open,  it will pick up too may noise and Amp it, feed back to input via radiation, lead to oscillating. But this is not true when it deploy with a mixer as input.  so to check if it was oscillating should pull the input down, i,e, via 10K resistor.

Really Progress to make it stable enough and eliminate the AC degeneration

[update @6/25  2012]

I got some suggestion @www.crystalradio.cn, I should try an IF transformer which had a primary tap.  I don't know why at that time, but tapped primary make my IF system is UN-conditionally stable.  eliminate all the AC degeneration both R9&R4. and free to increase the transistor's gm.   All of them is amazing.

Why?  All will continue in IF.B0: Gain Vs Stable,  Dig I.F.:why tap stabilize Tuned Amp,  Dig I.F.: Radio Traditional BJT IF strip,